The microstructure and composition distribution of the material are significantly influenced by the solidification process, thus affecting the final properties of product. As the initial stage of solidification process, nucleation greatly influences the order of subsequent phase formation, composition, grain size and distribution, and has a non-negligible impact on the final properties of the material. The magnetic field is an important method to modify the solidification process. Therefore, the study of nucleation under magnetic fields is of great importance to tune the microstructure during metal solidification via applied magnetic field. However, the current researches are very limited, mostly focused on the macroscopic experimental phenomena, which can only provide qualitative explanations, lacking in-depth investigation on heterogeneous nucleation process and magnetic field mechanism at the microscopic scale. In this paper, research status of nucleation theory, models and nucleation processes is introduced, followed by the current situation on solidification processes of metals under magnetic fields, and the development directions are proposed.